Microplastic shape, concentration and polymer type affect soil properties and plant biomass

Abstract

Microplastics are an increasing concern in terrestrial systems. These particles can be incorporated into the soil in a wide range of shapes and polymers, reflecting the fact that manufacturers produce plastics in a variety of physical and chemical properties matching their intended use.Despite of this, little is known about the effects that the addition into the soil of microplastics of different shapes, polymer type and concentration levels may have on soil properties and plant performance.To fill this gap, we selected four microplastic shapes: fibers, films, foams and fragments; and for each shape we selected three microplastics made of one of the following polymers: polyester, polyamide, polypropylene, polyethylene, polyethylenterephthalat, polyurethane, polystyrene and polycarbonate. In a glasshouse experiment, each microplastic was added to a soil from a dry grassland at a concentration of 0.1%, 0.2%, 0.3% and 0,4%. A carrot (Daucus carota) plant grew in each pot during four weeks. At harvest, shoot and root mass, soil aggregation and microbial activity were measured.Our results showed that all microplastic shapes increased shoot and root masses. As concentration increased, microfibers increased plant biomass probably as fibers may hold water in the soil for longer. In contrast, microfilms decreased biomass with concentration, likely because they can create channels in the soil that promote water evaporation affecting plant performance. All microplastic shapes decreased soil aggregation, probably since microplastics may introduce fracture points in the aggregates affecting their stability and also due to potential negative effects on soil biota. The latter may also explain the decrease in microbial activity with, for example, polyethylene films. Similar to plant biomass, microfilms decreased soil aggregation with increasing concentration.Our study tested the microplastic shape mediation and dissimilarity hypotheses, highlighting the importance of microplastic shape, polymer type and concentration when studying the effects of microplastics on terrestrial systems.

Microplastic Shape, Polymer Type, and Concentration Affect Soil Properties and Plant Biomass

“…For instance, contrary to our results, Qi et al (2018) found that polyethylene films did not affect biomass of a wheat crop, van Kleunen et al (2019) found a negative effect of microplastics on plant biomass, while Lozano and Rillig (2020) found that PES fibers may increase biomass of some plant species while decreasing that of others in a grassland community. Likewise, contrary to our results, microfibers may rather promote soil aggregation at the plant community level ( Lozano et al, 2020 ). As plant species can respond differently to microplastic addition, more research is needed in order to understand the effects of shape, polymer type, and concentration levels on plant performance and soil properties in a wide range of plant species and in a variety of soils.…”

“…Finally, as microplastics are ubiquitous around the globe, any effects of microplastics on plant-soil systems would have consequences not only in grasslands but also in different ecosystems worldwide. For instance, drylands, one of the largest terrestrial biomes that cover 41% of Earth’s land surface and that supports over 38% of the global human population ( Reynolds et al, 2007 ), characterized by its water scarcity, can be highly threatened with an increasing of microplastic concentration in the soil, especially as microplastics can exacerbate the negative effects that other global change factors as drought have on plant communities ( Lozano et al, 2020 ), soil properties, and ecosystem multifunctionality ( Lozano et al, 2021 ). This in turn may affect ecosystem services ( Díaz et al, 2018 ; Manning et al, 2018 ) and thus impact various aspects of human well-being.…”

“…For instance, microplastic granules of ethylene propylene at 5% may decrease plant biomass likely linked to its polymer composition ( van Kleunen et al, 2019 ), while polyester (PES) fibers at 0.4% may have the opposite effect as microfibers can enhance soil water content and soil aeration ( de Souza Machado et al, 2019 ). PES fibers can also increase soil aggregation as they may help to entangle soil particles ( de Souza Machado et al, 2019 ; Lozano et al, 2020 ), while opposite effects are detectable for, e.g., polyamide (PA) fibers ( de Souza Machado et al, 2019 ). Likewise, microplastics can affect soil microbial activity as they can increase mortality and histological damage in soil macro-organisms ( Huerta Lwanga et al, 2016 ; Rodriguez-Seijo et al, 2017 ), and decline richness and diversity of bacterial communities as seen with polyethylene films ( Huang et al, 2019 ; Fei et al, 2020 ).…”

“…We thank Sabine Buchert, Anja Wulf, Gabriele Erzigkeit, Jenny M. Ospina, Andrea C. Gundry, and Emily Magkourilou for their assistance in the experimental setup and data collection. This manuscript has been released as a pre-print in bioRxiv ( Lozano et al, 2020 ).…”

Microplastic effects on carbon cycling processes in soils

“…In addition, key plant symbionts, such as root-colonizing mycorrhizal fungi, might be affected by MPs or their effects on soil physicochemical properties. In some cases, resulting effects of MP on plant growth have been positive [ 20 , 21 ], but there are also reports of negative effects [ 22 ]. Such differences are explained by the fact that different MPs (including their chemical additives, some of which might be toxic), soils, and plants were used in these different studies, but it is not clear how each of these factors contributed to observed effects.…”